This invention relates to a constant voltage output circuit and more particularly to a constant voltage output circuit for obtaining a constant voltage using a voltage level of an input power source given to a reference potential as its reference.
In an electronic circuit, a constant voltage output circuit is sometimes required in order to obtain a certain constant voltage, which is stabilized with respect to voltage fluctuation of a power source, using, as its reference potential, a power source potential (power source voltage level) given to a reference potential (ground potential) of the electronic circuit. Such a constant voltage output circuit must, first of all, have excellent electrical characteristics. At the same time, it must have a circuit arrangement which does not impose limitation on production techniques of semiconductor integrated circuitry since it may be formed in a semi-conductor substrate such as a silicon substrate together with other necessary electronic circuits.
A constant voltage output circuit such as shown in FIG. 1 would readily be devised as a simple circuit for obtaining a constant voltage by use of a potential at a power source terminal, which feeds a power source voltage to a reference potential of the circuit, as its reference potential, on the contrary. In this circuit the output of a series circuit consisting of a zener diode Z.sub.10 and a resistor R.sub.10 is received by an emitter follower circuit consisting of a transistor Q.sub.10 and a resistor R.sub.11 to obtain a constant voltage V.sub.out from the emitter. As the zener diode Z.sub.10 is connected on the side of the power source in this circuit, the constant voltage V.sub.out is obtained across the collector and emitter of the transistor Q.sub.10 using a power source voltage level V.sub.cc as its reference potential. The circuit of this type uses an npn transistor as an output transistor (Q.sub.10) for which a large current capacity is required. From the aspect of integrated circuit techniques, the use of the npn transistor is more advantageous than the use of a pnp transistor of a lateral construction calling for a relatively greater occupying area because it minimizes a space requirement in the semiconductor substrate. From the aspect of electric characteristics, however, this constant voltage output circuit is not free from a drawback in that it is not easy to obtain a low output impedance because the output impedance of the constant voltage output circuit relies upon the emitter resistor R.sub.11 that determines the operating current of the emitter follower circuit.